UNRULY Bathtub plug!

How in the heck do I get this drain plug out of the old tub? In the photo you can see I unscrewed a top cap. The rod in the very middle doesn't go up or down and seems stationary. The stopper unit around it moves up and down and has a collar underneath that slide up and down over top of the middle rod. But if you look under there, there's no screw or anything. I've tried turning the stopper both direction buy it only spins endlessly. I can't put it up and off either. I've tried everything I can think of! What's the secret?!